14 DAY TRIAL // A long standing debate about global warming whether is nothing but a natural occurring phenomenon or it is caused by human influences. The climate has suffered various dramatic changes in the past, long before the existence of human beings or technology. Thus, certain climatologists argue that the currently occurring global warming is not caused by human interference either.
The most important cause of climate change is astronomical: the Earth's orbit small disturbances, getting closer to the Sun of further away from the Sun, disturbances generated by the other planets. (The solar system isn't a perfect clockwork system, it is affected by various chaotic phenomena).
However, in addition to this main cause, there are various internal factors that contribute to the increase or decrease in global warming (or cooling). Nonetheless, these internal factors often have different, contradictory effects: for example the increase of CO2 causes a greenhouse effect which contributes to global warming, but the same increase favors the growth of more plants which in return produce more oxygen and water in the atmosphere, i.e. more clouds. Thus, more sunlight is being reflected from the Earth, which causes a decrease in global warming. It is thus very difficult to actually model Earth's atmosphere and seeing whether a certain factor has a positive or negative contribution to global warming. In this context, it is worth trying to prove whether or not global warming is caused (or favored) by humans on empirical rather than theoretical grounds.
This is exactly what oceanographer David Field and his colleagues tried to do in a research published in this week's issue of the journal Science. The Northeastern Pacific ocean contains several species of plankton. The species Field has studied (referred as foraminifera) are some amoeba-like organisms that live inside tiny shells. Some of these organisms thrive is warmer waters while others in cooler waters. Usually they live near the ocean's surface, but when they die they leave behind their shells which sink to the seafloor, often mixing with the sediment there to form distinct annual layers. In some areas these annual layers remain relatively intact for millennia. A good example is Field's study region in the 600-meter deep Santa Barbara Basin, off Southern California, where the oxygen-poor waters support very few bottom-dwelling animals that can disturb the sediments. As a result, Field was able to study fossilized shells in one- to three-meter-long sediment cores reaching back to some 1,400 years before the present. Counting the different species of foraminifera in each layer, he and colleagues discovered that many species of tropical and subtropical origin became more abundant after about 1925. This was consistent with previous studies that suggested an ocean warming trend beginning at about this time. Field's sediment cores also show that during the mid-1970s the tropical and subtropical species have become even more abundant, while the ones preferring cooler waters decreased. Again, this was consistent with other previous studies that have indicated a rapid warming and a dramatic change in eastern North Pacific ecosystems during this period. At this time, species of plankton, kelp, fish and seabirds that prefer warmer waters increased and species favoring colder conditions decreased.
However, what Field's study reveals, in addition to these other studies, is the kind of long-term data needed to clarify whether the warming trend and ecosystem changes are within the range of natural variability or, conversely, should be considered the result of human activities. "These data show that ocean warming has affected foram populations prior to the late twentieth century," said Field. "However, changes since the 1970s have been particularly unusual, and show that ocean ecosystems in the northeastern Pacific have passed some threshold of natural variability." Indeed, he says, the foram community found in the cores was unlike anything seen during the last 1,400 years. As a result, he says, one can argue that the ecosystem changes since the mid-1970s are best explained by recent warming.
Field said that most scientific data about the ocean have been collected during recent decades, after ocean temperatures and marine ecosystems had already begun to change. "It's a classic case of 'shifting baselines'--conditions that scientists think of as normal today might actually be atypical when you look back a few hundred years." However, Field's data, although provide some insight and support the idea of a human induced global warming, are still too short termed. For example, it is known that around 55 million years ago there was a dramatic and sudden global warming, called the 'Paleocene-Eocene Thermal Maximum' (PETM) which obviously had nothing to do with human activity. However, the studies on PETM also reveal that the today's levels of greenhouse gases are approaching the levels during the PETM. Field's data may be interpreted as showing that human activities are the cause for this tendency.
Pictures credits: David Field
